For example, a load such as a lamp or a motor mounted on a vehicle is connected to a battery via an electronic switch (for example, a MOSFET) and the driving and stopping operations thereof are controlled by switching the electronic switch between the turning-on and off states thereof. Further, when a failure such as a short-circuit occurs in the load and hence an overcurrent flows through the load-driving circuit, the load and a connection harness may be broken due to generated heat. Thus, in the related art, an overcurrent protection device is mounted so that when an overcurrent occurs in the load-driving circuit, the electronic switch is turned off immediately to thereby protect the electronic switch and the electric wires provided in the circuit from the overcurrent (see a Patent Literature 1, for example).
Further, when dead short occurs in the load-driving circuit, a counter electromotive force due to the impedance of the electric wiring is generated to thereby abruptly reduce the voltage on the power supply side. Thus, according to the method of detecting an overcurrent based on a comparison result between a load current and a reference current, there may arise a case that when the power supply voltage reduces a voltage equal to or lower than the operation range voltage of an operational amplifier, an overcurrent detection circuit itself can not perform its function and hence the electronic switch can not be operated normally.
Accordingly, as described in a Patent Literature 2, a method is proposed that when the power supply voltage reduces to a value lower than a preset lower limit due to the generation of a counter electromotive force, the respective electronic switches of a plurality of load-driving circuits are instantaneously turned off and a retry operation for turning the respective electronic switches on again upon the lapse of a predetermined time is executed. According to this method, when the reason of the voltage reduction is other than the dead short, each of the electronic switches can be maintained in an on-state as it is since the power supply voltage returns to the steady state due to the execution of the retry operation. On the other hand, when the dead short occurs in one of the circuits, since the electronic switch of the circuit causing the generation of the dead short can be latched on the off state, the electronic and the electric wires can be protected from overheat.
Further, the document discloses that at the time of executing the retry operation, the timings for turning the respective electronic switches on are controlled so as to have time differences thereamong (so that the respective electronic switches are not tuned on simultaneously) to thereby avoid that rush currents flowing into the respective load-driving circuits are not superimposed. In other words, at the time of executing the retry operation, the respective electronic switches are not tuned on simultaneously but turned on with time differences thereamong to thereby specify the circuit where the dead short occurs.